Fix CS0315 for non-IUtf8SpanFormattable types (bool, enum, custom structs)

[DJGosnell]

Summary

• Fixes CS0315 compilation error when using bool, enum, or custom struct parameters in [LogMessage] methods. The generator now resolves SerializationKind per parameter at classification time by checking IUtf8SpanFormattable on the type symbol, emitting the correct write path for each type.

Reason for Change

The source generator unconditionally emitted writer.WriteFormatted(value) for all non-string parameters, which has a generic constraint of IUtf8SpanFormattable. Types like bool, enums, and custom structs do not implement this interface, causing CS0315 in generated code.

Impact

• bool parameters now emit zero-allocation UTF-8 literals: writer.Write(value ? "true"u8 : "false"u8)
• enum and custom struct parameters fall back to writer.WriteString(value.ToString())
• Types implementing IUtf8SpanFormattable (int, double, decimal, etc.) are unchanged
• Format specifiers on non-formattable types now emit .ToString("spec") instead of failing
• All generator model types (ParameterInfo, LogMethodInfo, TemplatePart, etc.) made internal — they were incorrectly public

Migration Steps

None required. This is a bug fix — previously broken code will now compile.

Performance Considerations

• bool: improved — zero allocation via UTF-8 literals instead of ToString()
• enum/custom structs: ToString() allocation is unavoidable without IUtf8SpanFormattable
• All IUtf8SpanFormattable types: no change

Security Considerations

None.

Breaking Changes

• Consumer-facing: None. Generator model types were public but not usable outside the analyzer — making them internal matches their intended usage.
• Internal: ParameterInfo gained a SerializationKind property (additive).